1. Field of the Invention
The present invention relates to a fixing device to fix an unfixed toner image on a recording medium and an image forming apparatus having such a fixing device.
2. Description of the Background Art
In general, a fixing device is provided in an image forming apparatus, such as a copier, a printer, a facsimile machine, a multifunctional machine having functions of these machines, etc., that employs an electrophotographic system to fix a toner image onto a recording medium, such as a paper sheet, etc. The fixing device includes, for example, a fixing roller with an internal heater and a pressing roller that presses against the fixing roller. The fixing device fixes a toner image onto the recording medium by conveying the recording medium through a nip formed by the fixing roller and the pressing roller pressing against each other.
To ensure stable fixing performance in this type of the fixing device, the temperature of the fixing roller needs to be maintained at a prescribed target temperature. Therefore, a temperature detector is generally provided to detect temperature of the surface of the fixing roller that controls the heater based on the detected temperature. As a method of controlling the heater, a so-called on/off control system is known in which the heater is turned on when the temperature detected by the temperature detector is lower than the prescribed target temperature and turned off when the detected temperature is higher than the prescribed target temperature.
However, using only on/off control the temperature of the fixing roller sometimes deviates significantly from the target temperature. Accordingly, an image forming apparatus described, for example, in Japanese Patent Application Publication No. 2008-122757 (JP-2008-122757-A) executes PID (Proportional, Integral, and Differential) control to minimize a difference (i.e., a temperature ripple) between a target temperature and a fixing roller's temperature in addition to on-off control. PID control is a method realized by combining proportional, integral, and differential calculations with a prescribed control algorithm, so that multiple parameters are optimized in accordance with the discrepancy between detected and target temperatures.
Further, to control temperature during a warm-up stage, various methods have been proposed as described, for example, in Japanese Patent Publication Nos. 2002-304090 (JP-2002-304090-A), 2004-78181 (JP-2004-78181-A), and H08-190292 (JP-H08-190292-A).
Specifically, JP-2002-304090-A employs the following relation: Standby temperature<Job start time control temperature<Job temperature, wherein the warm-up temperature represents a target temperature during a warm-up stage, the job start time control temperature represents a reference for starting a job, and the job temperature represents a target temperature during a job. Hence, an increase in the temperature of the fixing roller during the warm-up stage and conversely a decrease therein during a job runtime is minimized to provide uniform temperature at a central portion (of the fixing roller.
JP-H08-190292-A describes an approach in which a power turn-on time for supplying power to the heater is corrected in accordance with a voltage fluctuation detected during the warm-up stage to suppress the variation in temperature ripple that is generally caused by the voltage fluctuation.
Further, a system configured to rotate and heat the fixing roller during the warm-up stage is known that maintains the fixing roller at a given temperature, for example. In such a fixing device, however, a problem occurs as described below with reference to FIGS. 17 and 18.
Specifically, FIG. 17 is a diagram that shows one example of a change in the temperature of the fixing roller when the heater is controlled using the above-described on-off control method. FIG. 18 is a diagram that shows an actual temperature waveform obtained from the fixing roller of FIG. 17. As there shown, since the heater is controlled to turn on in accordance with a percentage of a ON time (hereafter simply referred to as a “ON duty”) of a given control cycle, a ON duty of about 100% is used when the surface temperature at the center of the fixing roller is lower than the target temperature, whereas the ON duty is 0% when the surface temperature at the center of the fixing roller is higher than the target temperature.
Further, in such a situation, the fixing roller is stopped after it is rotated for a given time period in the warm-up stage. However, the rate of surface temperature increase at the center of the fixing roller is different when the fixing roller is rotating from when it stops rotating.
Specifically, the surface temperature at the center of the fixing roller does not increase as much when the fixing roller is stopped as when the fixing roller is rotating. As a result, the heater stays on longer than necessary when the fixing roller is rotating, and because of this the surface temperature at the center of the fixing roller overshoots the target temperature after the fixing roller enters the non-rotating state. When a paper sheet bearing toner passes through the fixing device under such an overshoot condition, the toner on the paper sheet is liquefied and cohesion thereof decreases due to its high temperature, thereby sticking to the fixing roller instead and causing a so-called high-temperature offset.
Further, even when the fixing roller is in the non-rotating state, but the target temperature has never been exceeded after the warm-up stage is entered, the overshoot again occurs frequently. This is because there is a time lag between when the heater is activated and when heat thereby generated actually increases the surface temperature of the fixing roller.
Hence, in a fixing device that heats the fixing roller while rotating it during the warm-up stage, the overshoot generally occurs after the fixing roller enters the non-rotational states from the rotational state or when a temperature of the fixing roller has never reached the target temperature after the warm-up stage is entered, and consequently a high-temperature offset more likely occurs as a problem.
However, temperature of a fixing roller is not controlled in a conventional fixing device based on rotation of the fixing roller and that of arriving of a temperature of the fixing roller at a target temperature. Yet conventionally no special countermeasures have been taken to suppress the above-described overshoot.